A common technique for recovering gold from gold-bearing ores is to leach the gold into an aqueous cyanide leach solution in which the gold is solubilized as gold-cyanide complex. In some instances, the gold is leached directly from the ore or a concentrate prepared from the ore. This is the case for many oxide ores. In other instances, prior to cyanide leaching the ore or ore concentrate is pretreated to effect a chemical change to enhance cyanide leach performance. For example, gold-bearing sulfide ores are often refractory to direct cyanide leaching. Therefore, prior to cyanide leaching, such refractory sulfide ores, or sulfide concentrates prepared from such refractory sulfide ores, are often subjected to an oxidative pretreatment to decompose sulfide minerals and thereby release gold for recovery during subsequent cyanide leaching.
The leach solution loaded with gold is often referred to as a “pregnant” leach solution. After the gold has been dissolved into a cyanide leach solution, the gold is then removed from the pregnant cyanide leach solution. This is typically accomplished by contacting the pregnant cyanide leach solution with activated carbon granules under conditions conducive to adsorption of the gold-cyanide complex onto the activated carbon granules. After the carbon granules are loaded with gold, the carbon granules are then separated from the now barren cyanide leach solution, which may be recycled to leach additional gold. Gold is removed from the loaded carbon granules by stripping the gold from the carbon granules using a suitable strip solution, such as for example a hot caustic solution. The gold is then removed from the strip solution, such as for example by electro-winning to prepare a crude metallic product called doré. The doré is subjected to further refining to high purity gold.
Cyanide leaching can be conducted either in a heap operation or in a reactor. In a heap operation, barren cyanide leach solution feed is applied to the surface of a heap of the mineral material to be treated. The cyanide leach solution percolates through the heap and leaches gold from the mineral material. Pregnant cyanide leach solution draining from the heap is collected and contacted with activated carbon to remove gold from the pregnant cyanide leach solution.
When cyanide leaching is conducted in a reactor, the mineral material to be leached is slurried with the cyanide leach solution in a reactor vessel or vessels for sufficient time for effective leaching of the gold. In a so-called carbon-in-pulp operation, following the cyanide leach, activated carbon is then contacted with the pulp in a series of vessels, with the activated carbon advancing through the series of vessels in a countercurrent fashion relative to advancement of the pulp. In a so-called carbon-in-leach operation, the leaching is conducted in the presence of the activated carbon, so that cyanide leaching and adsorption onto activated carbon occur simultaneously in the same vessels.
The cyanide leach process is an industry standard that works well in many situations. There are, however, situations when implementation of a cyanide leach is difficult or impractical.
One problematic situation involves the processing of refractory sulfide gold ores in which the gold is locked within one or more sulfide mineral from which the gold is generally not amenable to recovery by direct cyanide leaching. As noted, these ores, or concentrates made from such ores, are frequently subjected to an oxidative pretreatment prior to cyanide leaching. During the oxidative pretreatment, at least a portion of sulfide sulfur in the sulfide mineral is oxidized, resulting in decomposition of sulfide minerals and release of gold. The gold released from the sulfide minerals remains with the solids following the oxidative pretreatment, and the solids are then leached with a cyanide leach solution to dissolve the gold. One oxidative pretreatment technique is bio-oxidation, in which sulfide sulfur in the ore or concentrate is oxidized as a result of microbial activity. Another oxidative pretreatment technique is pressure oxidation, in which the ore or concentrate is subjected to oxygen gas at high temperature and pressure in an autoclave. Yet another oxidative pretreatment technique is oxidative roasting of the ore or concentrate.
A problem with cyanide leaching of the residual solids following oxidative pretreatment is that the residual solids are often highly acidic, while the cyanide leach must ordinarily be conducted at an alkaline pH. As a consequence, it is necessary to neutralize the solids prior to cyanide leaching. This neutralization typically requires adding large quantities of lime or some other neutralizing reagent to the solids, and significantly adds to the expense and complexity of the operation. In the case of bio-oxidation that has been performed in a heap, neutralizing the solids requires removing the heap following bio-oxidation, neutralizing the oxidized solids by mixing the solids with lime or some other neutralizing reagent, and then depositing a new heap of the neutralized mixture for the cyanide leach. Removing a heap, neutralizing solids and depositing a new heap following bio-oxidation to facilitate cyanide leaching significantly add to the cost and complexity of the gold recovery operation. Also, even after the addition of a neutralizing agent, the solids typically still contain significant sulfide sulfur, the presence of which can complicate gold recovery operations. For example, some amount of sulfide sulfur may continue to oxidize during cyanide leaching operations, and even low levels of such oxidation may cause significant material handling problems. One such material handling problem is that oxidized sulfur can react with calcium from the neutralizing agent (e.g., from the lime addition) to form insoluble gypsum, that can plug pores in the heap, resulting in localized reductions in heap permeability during cyanide leaching. As another example, cyanide lixiviant may react directly with remaining sulfide sulfur, resulting in high consumption of cyanide.
Another problematic situation involves processing sulfide gold ores that have only a moderate sulfide mineral content. As opposed to refractory sulfide gold ores, these moderately sulfuric ores typically have a lower sulfide sulfur content and an appreciable portion of the gold is frequently recoverable by direct cyanide leaching. Cyanide leaching is, nevertheless, operationally difficult because these ores tend to be highly acidic, and often produce significant quantities of sulfuric acid as sulfide minerals oxidize during storage and during cyanide leaching operations. The need to neutralize such ores for cyanide leaching presents a significant problem.
Yet another problematic situation involves processing either sulfide or oxide gold ores that contain appreciable quantities of copper in a form that is susceptible to dissolving into the cyanide leach solution along with the gold. The presence of significant quantities of dissolved copper in the cyanide leach solution complicates gold recovery and increases processing costs. Furthermore, it is necessary to destroy copper cyanide for disposal, further increasing processing costs. Although it is sometimes possible to preleach copper from the ore, such as with a sulfuric acid solution, the preleached ore will still require neutralization prior to cyanide leaching. Also, if the ore is being processed in a heap operation, following the acidic preleach it is necessary to remove the heap, neutralize the solids and deposit a new heap for the cyanide leach, presenting problems similar to the situation with bio-oxidation of refractory sulfide ores, as discussed above.
Still a further problematic situation involves processing of either sulfide or oxide gold ores that contain appreciable quantities of organic carbonaceous material that has an affinity to adsorb the gold-cyanide complex during cyanide leaching. Such refractory carbonaceous ores are frequently referred to as “preg-robbing” ores, because available gold is “robbed” from the pregnant cyanide leach solution by the organic carbonaceous material. Several pretreatment techniques have been proposed to reduce or eliminate the preg-robbing ability of the organic carbonaceous material. These pretreatment techniques typically leave the ore in an acidic state requiring neutralization prior to cyanide leaching. As an alternative, thiosulfate lixiviants have been used to leach gold from such refractory carbonaceous ores without first pretreating the ores to destroy the preg-robbing ability of the organic carbonaceous material. The resulting gold-thiosulfate complex is less susceptible to being adsorbed on organic carbonaceous material than gold-cyanide complex. As with cyanide, however, such thiosulfate leaching operations must generally be operated at an alkaline pH, which can require significant neutralization prior to the thiosulfate leach, depending upon the specific ore being processed and the specific processing operation being employed.
In addition to the foregoing, there continues to be increased regulatory restrictions placed on the use of cyanide for gold leaching operations. There has, therefore, been interest in the gold mining industry to identify alternative processes for leaching gold that use lixiviants other than cyanide. For example, the potential use of thiourea and thiosulfate lixiviants has received considerable attention. The use of thiourea, however, is typically not practical due to high thiourea consumption caused by a high susceptibility of thiourea to oxidative degradation. Greater success has been achieved with the use of thiosulfate lixiviants, but, as noted, thiosulfate leaching operations generally must be conducted at an alkaline pH, presenting the same technical problems in many situations as noted previously with respect to cyanide leaching. Moreover, removal of gold from pregnant thiosulfate leach solutions is considerably more difficult than gold removal from pregnant cyanide leach solutions, because gold-thiosulfate complex does not readily adsorb onto activated carbon granules. Still other lixiviants have been suggested as alternatives for cyanide, but have not been investigated to a large extent, and practical implementation has been uncertain.